1. Technical Field
This invention relates generally to computer controlled photographic film analyzing systems employing digital electronics and video displays, and more particularly, to a method for automatically and interactively developing contrast tables for use in such systems.
2. Background Information
In the professional and consumer photofinishing industry, there is a trend towards use of hybrid systems that merge silver halide photography and video imaging, to analyze developed film images and determine optimum exposure values for printing of such images. Generally, in such hybrid analyzer systems, a photographic film image is scanned to produce electrical signals proportionate to film density along the scan pattern. The electrical signals are processed and then inputted to a video display device such as a CRT monitor to present a video picture of the film image. The video picture is compared to a reference print of the image and the contrast of the video picture adjusted, in a calibration mode, until a match in visual appearance is achieved. The calibrated analyzer can then be used to view film images desired to be printed, to adjust the balance of the viewed images and to determine optimum exposure values (or times) for printing of said images on paper. The key to successful operation of such an analyzer is to ensure that the video picture is a true representation of what the printed image will look like when printed on particular paper.
Hybrid analyzer systems generally incorporate various subsystems including sensors, scanners, signal processors and video monitors. Sharpness, resolution, color, neutral scale and red (R), green (G), and blue (B) contrasts are examples of some critical color monitor parameters that are required for proper pictorial video imaging and reproduction. The hardware video drivers for each of the primary RGB electron guns in the video monitor require input signals that are tailored to control the electron flow to produce desired RGB contrast scales and acceptable pictorial video images. Each monitor input signal needs to be appropriately compensated and corrected for any subsystem contrast distortions from Aim, i.e. a reference print. Today, this is generally achieved through the use of RGB contrast tables.
An early hybrid analyzer system known as the Video Color Negative Analyzer employed analog circuitry to develop visually acceptable monitor contrast for pictorial video images. However, in such hardwired analog systems, it was difficult to accommodate changes in film type or paper. More recently, digital hybrid analyzer systems such as the Kodak.RTM. Professional Video Analyzer Computer XL (hereinafter referred to as PVAC) have been developed to provide more flexibility in the handling of different film and paper types.
The PVAC scans negatives and slides and, using digital communications and processing transmits the image to a large video picture display. The PVAC is used in professional laboratories where film analyzing is critical as it effectively provides the correct set of RGB printer times for each negative and consequently minimizes photographic paper waste due to unacceptable prints. Neutral density (D) and RGB analyzer printing exposure numbers are the output of the PVAC used to control the RGB exposures in a subsequent printing station.
In digital analyzer equipment such as the PVAC, system contrast control and fine tuning is important to provide acceptable overall RGB tone scales of pictorial-video images. Poor video images can result if, for example, signal drift or hardware parameter variations exist between members of the same product line of equipment. Elaborate hardware feedback controls could be used to compensate for drift, but would be costly to implement. Furthermore, even if such hardware feedback control was employed, contrast fine-tuning could still be necessary. End user tastes often affect the monitor parameter settings for the video pictorial image, especially during the PVAC calibration process, where a video pictorial "match" to an Aim print is required. A match is attained when both images are subjectively deemed the same for overall color and density (i.e. image and print balance) and RGB tone scales.
In prior PVAC's, this match was sought to be achieved through the provision and storage in memory of multiple sets of predetermined contrast tables. A contrast table consists of ordered sets of consecutive integer input values and associated integer output values which define a contrast or tone scale.
In early versions of PVAC, multiple sets of discreet contrast tables (one for each color channel) had to be stored in memory. This approach either required large capacity memory or limited the number of contrast tables available to the user. Furthermore, contrast table selection required use of the video monitor screen and therefore necessitated flipping between the selection screen and the video picture presentation. Finally, the predetermined contrast tables did not always allow for the desired fine tuning of contrast scales.
A need thus exists for an improved contrast table generation method which allows for finer contrast adjustment, improved resolution, greater range of contrast settings and uninterrupted on-line, real time viewing of the contrast adjusted video picture.